This present invention relates generally to turbines and turbine blades and more particularly, to tip-shrouded turbine blades and associated cellular seal structures.
An axial gas turbine stage consists of a row of stationary blades followed by a row of rotating blades or buckets in an annulus defined by the turbine casing or stator. The flow is partially expanded in the vanes which direct the flow to the rotating blades where it is further expanded to generate required power output. For safe mechanical operation, there exists a minimum physical clearance requirement between the tip of the rotating blade and the casing or stator wall. Honeycomb strips on the casing wall are generally used to minimize running tip clearance of the rotating bucket at all operating conditions. To achieve tighter clearance, a rail on the tip shroud is allowed to rub and cut a groove in the honeycomb strip during transient operations. The shape and depth of this groove depends on the rotor dynamics and thermal behavior, i.e., differential radial and axial thermal expansion of the rotor and casing.
The high energy flow escaping over the bucket tips and its subsequent interaction with the downstream main flow is one of the major sources of loss in the turbine stage. Typically, these tip clearance losses in turbines constitute 20 to 25 percent of the total losses within a given stage. Due to the inherent shape of the groove cut in the honeycomb seal structure, the overtip leakage flow turns downward (i.e., radially inward) and penetrates deep into the main flow path causing excessive mixing losses. Accordingly, any design which minimizes this mixing loss will improve the turbine stage efficiency. In addition, the turning inward of high temperature, overtip leakage flow due to the groove shape and honeycomb seal configuration, causes the tip leakage flow to touch the aft side of the bucket tip shroud, exposing it to a relatively hotter operating environment compared to a non-grooved seal configuration. Since the bucket shroud is one of the life-limiting components of the turbine machine, any design which reduces shroud temperature will enhance bucket life.